The present invention relates to a reaction vessel, and particularly to a reaction vessel which is suited for regenerating adsorbents. There is already known a wide variety of reaction vessels of different types, sizes, shapes and for different purposes. The present invention is concerned with that type of a reaction vessel in which a mixture of particles of at least two materials having different bulk densities is to be confined in form of a bed which continuously or intermittently advances to a lower outlet through which it is withdrawn or discharged, and which is continuously replenished by the two materials from above under the formation of a cone at the upper region of the bed.
The vessels of this type are particularly suited for the regeneration of absorbents which have been previously used for adsorbing deleterious or noxious substances from different media, such as gases or liquids, and which have been charged with such substances. Such adsorbents, such as activated carbon or other carbon-containing substances, can be regenerated so as to regain their original adsorbing properties, by being heated to or above a regeneration temperature. Among the methods which can be used for heating the charged adsorbent to the regeneration temperature is the one which is utilized in the arrangement of the present invention, which resides in mixing the charged adsorbent with a particulate regenerating material which is at a temperature higher than the regeneration temperature needed for regenerating the adsorbent. Obviously, upon admixing the particles of the adsorbent with those of the regenerating material, such as, for instance, sand, heat exchange will take place between the particles, as a result of which the particles of the adsorbents are brought up to or above the regeneration temperature and the adsorbed substances are expelled therefrom and escape from the bed in the form of gases.
A vessel of this type is already known, and has a lower outlet for the mixture of the regenerating material with the regenerated adsorbent through which the mixture is discharged to be subsequently separated into its components, if so desired, for separate re-use. This separation, if contemplated, is performed in a conventional manner and will not be discussed. The conventional reaction vessel of this type has two coaxial upper inlets, one for the charged adsorbent to be regenerated, and the other for the regenerating material, through which these particulate materials are admitted into the interior of the reaction vessel and on top of the bed of the mixture which is confined in the reaction vessel and gradually descends toward the outlet as the mixture is being discharged through the bottom outlet. The bed is continuously replenished with the two materials through the two inlets, and a cone forms at the upper region of the bed, the slope of which cone will depend on the angle of repose, or, in other words, on the friction between the particles, of the two materials.
One of the most important requirements of such reaction vessels is that the two materials be thoroughly and intimately admixed with one another, which is a very difficult goal to achieve in view of the fact that the particles of the two materials have different bulk densities or specific weights, because heat exchange is to take place between the particles of the two materials, and because gases are liberated from the particles of the adsorbent being regenerated or desorbed. A uniform distribution of the adsorbent which is to be heated to the regeneration temperature in the particulate heat carrier, such as hot sand, is imperative in order to achieve a uniform temperature distribution throughout the bed and thereby a uniform and complete regeneration or desorption of the adsorbent.
Experience with the above-described conventional reaction vessel has shown that it is possessed of several disadvantages. One of the main drawbacks of such a reaction vessel is that the flow of the particles of the two materials in all directions within the reaction vessel is unimpeded. This, in turn, brings about other disadvantages. First of all, the particles of the charged replenishment adsorbent do not readily become immersed into the bed once they are introduced into the reaction vessel at the top of the above-mentioned cone at the top of the bed. This situation is attributable to the fact that the particles of the adsorbent have a lower specific weight than the particles of the regenerating material, so that they will naturally tend to remain at the surface of the cone. Secondly, as the bed descends toward the bottom outlet, and the regenerating material and the adsorbent are being replenished from above, the particles of the adsorbent and of the regenerating material will flow down the slope of the previously formed conical upper region of the bed, during which downward and radial flow the adsorbent particles, due to their low specific weight, will tend to float toward the upper surface of the conically shaped upper region of the bed. Furthermore, as already mentioned above, gases are being desorbed from the particles of the adsorbent during the regeneration of the latter, which gases are collected above the upper surface of the bed, after having flown through the bed from the lower regions thereof in countercurrent to the descent of the bed toward the outlet. This counterflow of the gases by itself will tend to entrain the lighter particles of the adsorbent and carry them with itself toward the upper surface of the conical upper region of the bed. In addition thereto, the penetration of the gases which have been desorbed from the adsorbent particles through the bed, and the escape of such gases from the bed into the gas-collection space from which the gases are withdrawn or exhausted results in the formation of turbulent flow conditions at the upper surface of the conical upper region of the bed which further enhances the rising of the low specific weight adsorbent particles to the upper surface of the conical upper region of the bed. These turbulent conditions with attendant formation of stray and eddy currents will tend to de-mix rather than admix the particles of different bulk densities. The net result of the above-discussed conditions in the interior of the reaction vessel is that the absorbent particles will have a tendency to accummulate at the upper surface of the conical region of the bed, and slide over such upper surface in the radially outward direction more readily than the particles of the regenerating material. Thus, a larger concentration of the adsorbent particles will be found at the foot of the conical upper region of the bed than in any other region, such a concentration being much higher than that which would correspond to the statistical distribution, and is also much higher than what would correspond to the relative amounts of replenishment adsorbent and replenishment heat-carrying regenerating material. This situation is disadvantageous in two respects. On the one hand, proportionately less regenerating material is available for heating the adsorbent in the radially outward regions of the bed, owing to the higher concentration of the adsorbent in the mixture in such regions. On the other hand, heat exchange will also take place between the regenerating material and the walls of the reaction vessel in such radially outward regions, so that even less heat is available for regenerating the adsorbent particles in such radially outward regions than there is in the inward regions of the bed. For these reasons, it is necessary, in the conventional reaction vessel, to use an inordinate amount of heat for regenerating the adsorbent even in the radially outward regions of the bed by either introducing disproportionately large quantities of the heat-carrying regenerating material, or by heating such regeneration material to a much higher temperature than actually necessary. When this is done, the energy consumption of the reaction vessel is substantially higher than that which would be otherwise needed if the concentration of the adsorbent were uniform throughout the bed. On the other hand, if only such an amount of energy was introduced into the vessel which corresponds to the statistical distribution of the adsorbent and regeneration material particles, the regeneration or desorption operation would be less than complete by the time of discharge or withdrawal of the mixture through the bottom of the vessel.